Terrane Accretion along the Western Cordilleran Margin: Constraints on Timing and Displacement

A fundamental controversy in North American Cordilleran tectonics is the timing and magnitude of large-scale terrane translation along the western Cordilleran margin during Late Cretaceous and Eocene time. A major discrepancy exists between tectonic models derived from geophysical (paleomagnetic) and geologic data sets. Paleomagnetic data consistently indicate large-magnitude translation of allochthonous terranes along the continental margin between ca. 90 and 55 Ma, whereas geologic data suggest a significantly smaller degree of displacement. Reconstruction of the tectonic evolution and paleogeographic configuration of the western Cordillera margin, particularly during the Late Cretaceous and Eocene, requires knowledge of the timing and magnitude of terrane translation.

The North American Cordillera is an ideal laboratory for examining the conflicting geological and paleomagnetic data sets, and this controversy has major implications for other orogenic belts. If multidisciplinary geologic correlations and terrane linkages in the North American Cordillera are suspect, then established geologically based tectonic models in other orogenic belts worldwide may require reassessment. Conversely, if paleomagnetic analyses in the North American Cordillera are called into question, then paleomagnetic reconstructions established in other orogenic belts or for different time periods become equally suspect. The recurring nature of the discrepancy throughout the Cordillera makes it clear that the problem is systematic, and resolution of the controversy requires examination of the processes and methods used to formulate tectonic reconstructions.

The Penrose Conference "Terrane Accretion along the Western Cordilleran Margin: Constraints on Timing and Displacement" was convened June 1825, 1999, in Winthrop, Washington, to evaluate the quality and limitations of data sets contributing to the translation paradox. Seventy-seven scientists from six countries provided geological expertise that ranged from Honduras to the tip of Alaska. The conference was designed to provide a multidisciplinary examination of differing data sets, and a broad range of expertise was brought to bear on the problem. A key element of the conference was the assembly of multiple investigators representing different disciplines from throughout the North American Cordillera focusing on a singular objective: constraining terrane translation along the western margin.

Format

The conference subdivided the topic into spatial and topical areas of discussion. The continental-scale scope of the problem, the widespread distribution of researchers, and the breadth of expertise represented required a format that would provide both a regional overview and topical discussions of the problem. The contentious nature of the debate raised the spectre of unproductive argumentation, and the format was designed to allow free-flowing discussion. Discussion sessions were structured to provide each researcher a chance to present, and to encourage interaction among participants. Posters were on display throughout the meeting and acted as the focus for many discussions.

The first discussion session was a spatial overview of Cordilleran terranes, subdivided into segments from south to north. The geologic setting of each segment was described by an established researcher. All other researchers working in the region then briefly described their area of interest and research specialty. The spatial overview very effectively set the tone of the meeting by introducing the regional geologic setting, highlighting outstanding controversies, introducing all researchers in each region, and providing a comprehensive review of the geologic setting of the continental margin for all participants.

The primary focus of the conference was a critical evaluation of the processes and methods used to examine accretionary tectonics and terrane translation. A variety of methods are used to evaluate the tectonic evolution of the Cordillera, and each method has inherent limitations and biases. The methodology sessions were organized by discipline and included paleomagnetism, geochronology, lithostratigraphy, biostratigraphy, structure, igneous and metamorphic geology, geophysics, and mantle dynamics. Participants in each session were encouraged to discuss the utility of their methods, including strengths and weaknesses, for constraining terrane translation. Each segment had a similar format: an overview talk that highlighted the benefits and limitations of the method, a brief (~5 minute) introduction into the methodological approach of each researcher in the discipline, a panel discussion, and dedicated time for poster discussion. Several researchers from each discipline formed a panel that fielded questions from all participants. The panel discussions were particularly effective in generating useful dialogue among participants. This format encouraged questions about strengths, limitations, and biases of research methods, and led to frank discussions about the positive and negative aspects of different approaches. The willingness of participants to discuss the limitations of their particular techniques was the principle reason the meeting was a success.

Field Trips

A field trip led by Ralph Haugerud and J. Brian Mahoney, from the accreted terranes of the San Juan Islands, across the crystalline core of the North Cascades, and into the allocthonous sedimentary basins of the Methow terrane introduced the complex geology, protracted geologic history, and domainal character of the North Cascades. Discussions concerned terrane correlations, constraining displacement across high-strain zones, terrane mobility along the continental margin, and the complexities of unraveling the regional geologic history.

On a mid-conference trip, participants examined terrane boundaries in the North Cascades and discussed structural and stratigraphic relationships along the Insular-Intermontane superterrane boundary. J. Brian Mahoney and Ralph Haugerud described proposed stratigraphic ties between the Cascades and the Methow terrane in Albian time, and between the Methow terrane and the western edge of the Intermontane superterrane in the Late Cretaceous.

Spatial Overview

A beginning spatial overview of Cordilleran terranes served to highlight similarities and differences in regional geology between Mexico and Alaska. Chris Scotese set the stage for the discussion by presenting recent paleogeographic reconstructions of the continental margin and describing ongoing problems in his reconstructions.

Fernando Ortega-Gutierrez began the overview with a description of the geologic setting and paleogeographic history of central Mexico. He described several transects through Mexico, introduced terrane nomenclature, and described the terranes of western Mexico as thin tectonic flakes imbricated onto a Precambrian Grenvillian suture. Tom Anderson followed with a sequential reconstruction of the Triassic to Eocene geologic history of northeastern Mexico. He emphasized the importance of Middle Jurassic sinistral displacement along the southwestern margin, and advocated a simple closure model for the Gulf of California.

Darrel Cowan reviewed the Cretaceous-Tertiary evolution of the Sierra Nevada system and described variations in the angle of subduction over the past 100 m.y. He emphasized the fundamental difference between thin-skinned Sevier-type deformation between 100 and 80 Ma and thick-skinned Laramide-type deformation between ca. 75 and 55 Ma. He brought the Sierran system into the debate by encouraging the group to consider what type of features we would expect to develop in an arc-forearc assemblage adjacent to a major transcurrent fault system. Jim Monger introduced the terrane assembly of the Canadian Cordillera and described the along-strike continuity of tectonostratigraphic units and the across-strike segmentation of the Cordillera into two "high-grade" belts (Omineca and Coast) and three supracrustal belts (Rocky Mountain, Intermontane, and Insular). He argued that the North America Cordillera is the key place to resolve the translation dilemma, owing to the north-south orientation of the margin and the east-west orientation of cratonal provinces and climatic zones, which should provide an ideal measure for terrane translation. Sarah Roeske described the tectonic evolution of the northern Canadian and Alaskan Cordillera. She emphasized the immense size of the translated terranes, major dextral strike-slip systems within some accretionary complexes (e.g., Chugach), and that the often-overlooked geology of Alaska is critical to the translation debate.

Processes and Methodologies

The examination of processes and methodologies began with a discussion of paleomagnetic techniques and data sets by Randy Enkin, who provided an excellent introduction to critical evaluation of data sets by explaining how to critically evaluate a paleomagnetic study. He examined the potential problems and bias inherent in paleomagnetic studies, and pointed out that a high-quality paleomagnetic investigation systematically evaluates potential problems, such as compaction shallowing, thermal overprinting, and other sources of experimental error. Panel discussions with paleomagnetic experts addressed the robustness of various paleomagnetic data sets, and there was a notable lack of consensus regarding the quality of various data sets and density of reliable data points throughout the Cordillera. There was a clear consensus that the paleomagnetic data are remarkably consistent, although resolution of the debate requires a substantially larger paleomagnetic data base.

Timing constraints on terrane accretion and translation were introduced by Dave Kimbrough and Bill McClelland. Kimbrough emphasized the magnitude of Late Cretaceous igneous events along the western margin, noting that the large scale of the Peninsular Ranges batholith and other features required tonalite production through partial melting of enormous volumes of upper-mantle peridotite. He described a major pulse of magmatism at 95 Ma in the batholith, which other researchers identified as an important time frame for Cordilleran evolution. McClelland argued that terrane linkages must be demonstrated by tight timing constraints on structural imbrication, stratigraphic overlap, tectonostratigraphic correlations, and pinning plutons. He described geologic constraints on development of the Coast Shear Zone in the northern Canadian Cordillera, intruded by the undeformed "great tonalite sill" between 80 and 55 Ma, placing an upper limit on the age of deformation in the shear zone. Jim Mortensen discussed constraints on movement on the Denali and Tintina fault systems, which are limited to ~450 km dextral offset in pre-Eocene time. Elena Centeno-Garcia discussed timing constraints on the evolution of the Guerrero terrane in central Mexico, which displays structural, stratigraphic, and isotopic evidence of two-phase deformational history, with major contractional and sinistral displacement between 208 and 156 Ma, and transpressional deformation of the Guerrero-Alistos arc prior to 83 Ma.

Sedimentologic and stratigraphic techniques are an important component of terrane analysis, and many participants cautioned about the reliability of proposed correlations. J. Brian Mahoney emphasized the need for multidisciplinary analyses in tectonostratigraphic analyses, including complimentary lithostratigraphic, paleontologic, geochemical, isotopic, provenance, and paleomagnetic linkages. Jim Haggart presented paleontologic data that argues for a northern origin of the Insular superterrane during Jurassic and Cretaceous time. Ralph Haugerud provided an insightful description of the assumptions inherent in geologic mapping. Individual presentations and panel discussion highlighted the need for reliable tectonostratigraphic correlations, the questionable reproducibility and utility of petrographic analyses, and the uncertainty over the interpretation of detrital zircon results.

Paul Umhoefer began the discussion of structural techniques in terrane analysis, emphasizing the very low preservation potential of structures developed during major translation. He used the modern San Andreas system as an example, where up to 1200 km of offset occurred, but only 300500 km of displacement is attributed to recognized faults. M. L. Crawford discussed the limitations of igneous and metamorphic analyses, which primarily provide information about mid- to lower-crustal processes. She emphasized the volume of magma produced in the Coast Mountains of the northern Canadian Cordillera between 100 and 88 Ma and 68 and 53 Ma, and suggested that these volumes required major orthogonal contraction during these periods. Ray Price described several kinematically linked tectonic episodes, including northeast- and southwest-vergent contraction, sinistral transpression, orthogonal convergence, dextral transpression, dextral transtension, and relative tectonic stability, all between 180 Ma and the present.

The final methodology session began with an overview of the recent major geophysical initiatives in the northern Canadian Cordillera. Ron Clowes described the initial results of the SNORCLE Lithoprobe transect, a crustal refraction profile from the Slave Province to the Insular superterrane. Linc Hollister described the implications of the ACCRETE profile for exhumation of the northern Coast Plutonic Complex and imaging of transpressive structures in the region. Ray Russo presented a dramatic model of the role of aesthenospheric mantle in terrane tectonics. He made the point that the terranes are probably not thin tectonic flakes, but contain lithospheric keels that control upper mantle flow. The model vividly illustrated the three-dimensional aspect of terrane translation. Basil Tikoff described probable far-field effects of terrane translation, emphasizing that the movement of thick-rooted arc terranes will have major regional effects, such as the formation of fold and thrust belts perpendicular to the margin. His description of a potential tectonic mechanism by which a major crustal block could move along the western Cordilleran margin sparked lively discussion among participants.

The final session of the meeting was a panel discussion on the recognition of large-scale terrane translation in the North American Cordillera. This session became a group forum on the possibility and plausibility of terrane translation during Cretaceous to Eocene time, and it was readily apparent that outstanding problems or inadequate data sets prevent resolution of the problem. Categories of the problem are:

better timing constraints are needed to define both the magmatic evolution of the arc system(s) and deformational episodes along the margin;

the quality of currently proposed lithostratigraphic and tectonostratigraphic correlations varies widely, and may require reexamination; provenance ties, particularly detrital zircon analyses, must be interpreted with caution;

paleomagnetic data sets must become more robust, with a higher density of data points throughout the Cordillera;

the scale of the problem must be recognized, which requires the incorporation of local and regional data sets into continental models; the apparent discontinuity of tectonostratigraphic units and fauna between Mexico and the northern Cordillera must be explained; the kinematics of plate motion must be reassessed; why Alaskan tectonic reconstructions differ from those to the south and where the Kula-Farallon triple junction was during the proposed translation also must be investigated; and

the geodynamics of terrane translation is poorly understood, and may require three-dimensional models of lithospheric motion; the kinematics and offset magnitudes in major shear zones are inherently difficult to assess and should be interpreted with caution; the geologic signature of terrane translation and why there is no structural evidence recording translation of the Insular superterrane must be examined.

The diversity of scientific perspectives, both disciplinary and geographic, allowed the translation debate to be examined from a broad, systematic viewpoint, and participants in the conference came away with new insights and understandings of the philosophy and methodology of other workers. The translation paradox will not be solved by a single researcher employing a single technique, but will be resolved by a community of workers engaged in multidisciplinary research.

Acknowledgments

This conference was made possible through the logistical and financial support of the Geological Society of America and the National Science Foundation (grant number EAR-9817962). Lois Elms (Western Experience, Inc.) handled logistical arrangements superbly, as usual. The conveners wish to thank Carrie Rowe and Michael Schmidt (University of Wisconsin-Eau Claire) for assistance during the conference, and Sam Campbell for providing a base of operations in Seattle. Nancy Amdahl (University of Wisconsin-Eau Claire) provided indispensable organizational assistance. Jim Peterson (Western Washington University) and Hugh Hurlow (Utah Geological Survey) assisted with the mid-meeting field trip in the Methow basin. The conveners extend their sincere gratitude to the participants, who provided the intellectual energy, scientific curiosity, professional camaraderie and positive attitudes necessary to make this conference a success.